docs: def union(self,
                sr,
                geometries):
          """
          The union operation is performed on a geometry service resource. This 
          operation constructs the set-theoretic union of the geometries in the 
          input array. All inputs must be of the same type.
          
          Inputs:
              geometries - array of geometries to be unioned (structured as JSON 
                           geometry objects returned by the ArcGIS REST API).
              sr - spatial reference of the input geometries.
          """
          url = self._url + "/union"
          params = {
              "f" : "json",
              "sr" : sr,
              "geometries" : self.__geometryListToGeomTemplate(geometries=geometries)
          }
          return self._get(url=url, param_dict=params,
                              securityHandler=self._securityHandler,
                              proxy_url=self._proxy_url,
                              proxy_port=self._proxy_port)